The process gases used in the semiconductor industry, such as greenhouse gases of the PFC type, are generally toxic, corrosive or even harmful to the ozone layer or cause a strong greenhouse effect. It is therefore necessary to treat the extracted gases before any gas is discharged into the atmosphere, so that only gases that are relatively harmless are discharged.
Hitherto, process gases have essentially been treated in gas treatment facilities situated on the delivery side of the pumping systems.
One first disadvantage with these known gas treatment facilities is the large volume they have to have in order to treat the substantial quantity of outgoing flow. These gas treatment facilities are therefore situated remotely, i.e. are placed on a lower storey in the semiconductor manufacturing plant, and are connected to the process chamber by lengthy and expensive ducting. The sheer volume of these gas treatment facilities makes it impossible for them to be sited near to the process chambers.
A second disadvantage is that the flow exiting the pumping systems contains not only the gases that have been extracted from the process chambers but also purge gases that are introduced into the vacuum sequence between the outlet of the process chambers and the delivery side of the pumping systems. These purge gases, which are added in order to protect the pumping systems, disrupt the effectiveness of the treatment and lead to an additional increase in the volume of the gas treatment facilities.
Document EP 1 538 656 proposes performing an at least partial treatment of the gases between two successive stages of a vacuum pump of the pumping system, by situating, actually inside the vacuum pump, a plasma source which breaks down the gases that are to be pumped, including the harmful gases that are to be treated. These decomposed gases can be recombined in the vacuum pump with a reactant gas, such as water or oxygen, and this prevents the recomposition of the gases that have been decomposed by the plasma and recomposes them according to a controlled chemical reaction into by-products that are less harmful.
Incorporating the gas treatment system into the pumping unit means that the harmful gases can be treated as close as possible to the process chambers and this reduces the size of the gas treatment system and avoids having to site it remotely on a lower storey of the semiconductor manufacturing plant.
The plasma is, for example, generated in an inter-stage ducting inside the vacuum stator. Using this arrangement, the gas treatment system does not have a disruptive effect on the effectiveness of the pumping.
However, it is found that the plasma is too closely confined in the inter-stage ducting. This confinement limits the path of the molecules that are to be treated. In order to achieve good treatment effectiveness it then becomes necessary greatly to increase the power of the plasma source. By way of example, in order to obtain an effectiveness of the order of 90% regarding the decomposition of CF4 gas in a conventional 200 mm oxide etching process, a plasma source with a power of the order of 3000 W is required. That leads to significant energy costs.
Moreover, the temperatures reached locally by the generation of the high-power plasma may lead to overheating of the body of the vacuum pump. This heating may cause the stator to expand, with the risk of causing it to deform, or in the worst case, even of causing the vacuum pump to seize. This overheating of the stator can also impair the sealing of the pump seals.